1. Technical Field
The present disclosure relates to a clock dithering process and apparatus for reducing electromagnetic interference in D/A converters, particularly audio D/A converters.
2. Description of the Related Art
The standards for radio-frequency interference in electronic apparatus have become increasingly stringent, especially in the automotive field, such as a radio, CD player or other audio or audio-visual device.
For example, according to such standards, the spectrum of a signal picked up by an antenna that has a specially regulated position relative to an apparatus undergoing an electromagnetic interference test, is below a preset threshold 1, e.g., a threshold having the profile as shown in FIG. 1.
On the other hand, the diagram of FIG. 2 shows a case in which the apparatus under test emits radiation above the preset threshold 1, which is picked up by the antenna. Due to noise, the antenna might pick up a plurality of peaks 2, 3 and 4 that exceed the limit defined by the threshold 1.
If the equipment under test is a digital electronic apparatus or a switching type apparatus, such as a class D digital amplifier, this high frequency noise is typically composed of clock harmonics of the equipment, and this noise can generate peaks, the shape and position of which are shown in FIG. 2.
One of the methods employed to reduce the amplitude of these peaks consists in frequency/phase modulation of the clock signal using a process commonly known as dithering, which is known to those of ordinary skill in the art and will not be described in further detail. For the purposes hereof, it shall be noted that the dithering process provides a clock signal having a frequency spectrum that no longer has one peak at the clock frequency and multiples thereof, but several peaks around the clock frequency, which can be reduced by as much as 20 dB.
This can be achieved by increasing the modulation index “m,” which is defined as the ratio of the maximum frequency deviation from the typical value (max ΔFclock) and the modulating frequency (fm);m=max ΔFclock/fm.
Due to this attenuation of peak values, the equipment under examination exhibits an improved response to the electromagnetic interference test. Thus, the clock dithering process is often used in equipment in which the clock signal need not be synchronized with other undithered clock signals external to the equipment.
In audio D/A converters, particularly those D/A converters whose power output is designed to drive loudspeakers, the dithering process would initially seem to require a considerable increase of circuit complexity with the additional risk of worsening the linearity performance and dynamic range of the system, which has to be very high in audio converters.
Typically, a clock dithering process in audio D/A converters is carried out by modulating a V.C.O. (Voltage Controlled Oscillator) with a modulating signal having an infrasonic modulation frequency, i.e., a low modulation frequency, typically of less than 20 Hz.
Nevertheless, increased difficulties occur if the clock signal of a D/A converter comes from an input clock signal through a Phase Locked Loop (PLL) because, when the input signal to the D/A converter is an audio signal, modulation of the clock signal is likely to worsen signal quality, due to intrinsic jitter caused by dithering.
Referring to FIGS. 3a and 3b, which show possible ideal (FIG. 3a) and actual (FIG. 3b) reconstructions of five samples C1-C5 with a sampling frequency above the dither frequency, it can be seen that, in the actual reconstruction (see FIG. 3b), the samples C2 and C4 are offset in time relative to the same samples C2 and C4 of the ideal reconstruction (see FIG. 3a). Such time offset causes distortion of the reconstructed signal, which leads to obvious imaginable consequences in terms of quality of the signal transmitted to the loudspeakers.
A PLL modulation example that does not meet the quality and simplicity requirements required of audio D/A converters is disclosed in U.S. Pat. No. 7,079,616 in which the dithering process is carried out by causing the division factor of the VCO feedback loop to be varied using an infrasonic triangular wave.